Acoustic wave devices are used as, for example, filters of wireless equipment or the like. It is required for the acoustic wave devices to reduce the absolute value of the temperature coefficient of frequency (TCF) of the pass bands of filters, the resonant frequencies of resonators or the like. It is known for surface acoustic wave devices that the absolute value of the TCF can be reduced by forming a dielectric film such as a silicon oxide film that has a temperature coefficient opposite to that of a piezoelectric substrate made of LiTaO3 (LT), LiNbO3 (LN) or the like, on the piezoelectric substrate.
Furthermore, in acoustic wave devices employing IDT electrodes, various measures are attempted to be taken in order to reduce high-order transverse mode spurious responses. For example, weighting may be performed by causing the width over which electrode fingers of the IDT electrodes alternate to be partially different. In this case, dummy electrode fingers are provided in a portion where adjacent electrode fingers do not alternate. With acoustic wave devices in which weighting is applied to the electrode fingers of the IDT electrodes, the sound velocity of acoustic waves (elastic waves) becomes discontinuous in an area where the acoustic waves propagate. As a result, undesirable effects such as scattering of acoustic waves that have passed through a portion where the sound velocity is discontinuous, or unintended mode conversion to other waves will occur, which increases loss in the acoustic wave devices.
Patent Document 1 described below suggests a method for scattering acoustic waves of transverse modes by causing a dummy electrode and a dummy portion of a cross electrode adjacent to the dummy electrode to have different lengths, thereby suppressing spurious responses.
Patent Document 1: International Publication Pamphlet No. WO2006/078001